TechNote#002 SIMPLIFIED DESIGN METHOD – DESIGN BY COMPARISON

INTRODUCTION

What follows is a simplified method of design and estimating of structural adequacy for use by shed fabricators/suppliers and other manufacturers/suppliers whose products are based on pre-engineered solutions.

These enterprises either have existing products that have a history of successful operation, or a range of products that have been demonstrated to be compliant with codes of practice either by calculation or physical testing.

The method of design presented involves a simple drafting exercise, drawing your known product to scale and comparing it against a custom product that is drawn to the same scale. This method is illustrated for the example of a portal frame gable shed and the size of c-section required for the frame. This method can be extended by drawing the end wall columns on the diagrams and determining the size of c-sections for end walls by comparison. The method can also be extended for other similar structural components, such as struts and bracing.

The basis of the method is the assumption that anything that fits inside the envelope of a structure that is known to be structurally adequate or compliant with codes of practice, is itself compliant.

The method is basically an extended and more formalised approach to what shed suppliers do all the time. That is shed suppliers usually submit a “standard” set of calculations to council for a shed that is larger than that which they propose to build. However, when they do so, they offer no explicit proof that their proposal is fit-for-purpose, structurally adequate or compliant with codes of practice.

In short proof of fitness-for-purpose is an informal and non-explicit declaration that the customers’ requirements are either equal to the specification in the submitted standard calculations or otherwise lie within the building envelope defined by such pre-engineered solutions. The latter being left to the building surveyor or private certifier to judge.

It is this leaving it to the building surveyor to make judgement, which usually results in rejection of the building application and a request for an engineer’s report or calculations. This request for an engineer’s calculations is a lazy way of requesting that the applicant demonstrate proof of structural adequacy and compliance with current codes of practice. In other words there is no requirement for an engineer.

To avoid such rejection and the delays that it generates, it therefore makes sense to make the judgements ourselves, and formally and explicitly provide proof-of-structural-adequacy.

The rest of this article illustrates a simplified method of doing so. The method is reliant on producing what I shall call a proof-template, from the available pre-engineered solutions that manufacturers/suppliers keep in-house.

The resulting proof-template/s can be used for design and more importantly determining the need for additional engineering calculations to achieve more economical specifications.

THE METHOD

As mentioned above the method is based on the assumption that a custom design is structurally adequate if its specification is enveloped by that of a standard design known to be structurally adequate.

Figure 1: Standard Design Compared Against Custom Design

This can be presented more clearly and explicitly by drawing a diagram to scale.

The diagram in figure 1 clearly shows that the custom design (in red) is smaller than the standard design. That is the custom design is enveloped by the standard specification.

Let us assume for a moment that structural mechanics has not been invented and codes of practice do not exist.

Our standard specification is merely a shed that we built previously and has not yet experienced any force that could destroy it. Thus we have historical evidence that it is acceptable. Our customers shed is a smaller shed built from sticks that keeps being blown over by strong winds, they are seeking our assistance to provide a stronger structure to protect it. Our larger standard shed does not, or at least has not experienced such wind damage. We can therefore surmise that if we build our larger shed over the top of the smaller shed then the smaller shed will then be provided with protection from the wind. Or at least it will not be blown over quite so often. But may be this idea is a waste of material. First, I don’t really want the bigger building, and two if the house of sticks is hidden under the steel shed why bother building it? So can I build a smaller building to a similar specification to the larger building?

The simple answer is probably an obvious yes. However, we first need to answer the question: Are there any reasons or is there any justification for the smaller building to require a larger section size than that of the larger building?

The answer to this question is not so simple. There are aspects of structural mechanics that may result in a larger section size for the smaller structure. And there are certainly explicit requirements in codes of practice that require smaller structures to be designed for loads of larger magnitude than those used for larger structures. In general however, as long as the height to span ratio (h/s) remains the same, making the building smaller, should reduce the effects of loads on the structure and result in a smaller section size. Unfortunately this is usually not the case with custom designs, heights tend to remain the same whilst, spans are reduced or increased.

Since we want to avoid structural mechanics the next best thing is to have two pre-engineered solutions. One that fully contains or envelopes our custom design, and a shed design for the next smaller size of c-section. This is shown in figure 2.

Figure 2, shows a more formal version of a proof-template and besides the building envelopes drawn to scale also contains the following information:

Title & Reference Number

Date Created

Frame Type

Roof Pitch

Frame Centres

Wind terrain category

Building Type

Report Number of Original Design

Size of C-section.

It should be noted that the custom design shown on the template has a different roof pitch to that of the standard designs. For those willing to accept high risks, the design would be accepted on this basis alone. For those who prefer greater evidence of adequacy, the different roof pitches would have to be considered, in which case additional calculations would be required. Or an alternative design chart based on such calculations.

However, it should be noted that increasing the roof pitch makes the roof experience a more positive pressure and behave more like a wall. Thus for the custom roof pitch we can determine an equivalent eaves height for the shed, to match the roof pitch of our standard designs.

For the example shown, draw the actual custom shed to scale, and then draw a roof pitch of 10 degrees from the apex, followed by extending the walls vertically until they intersect the lines of the roof. Then scale-off the new higher eaves height. Then use the proof-template to check compliance of this equivalent design. This is illustrated in Figure 3, for the steep roof example. Further illustration of equivalent eaves height is illustrated in Figure 4, for a skillion roof shed.

Further checks that can be made using this chart is comparing the equivalent eaves height with that of the standard design. If the eaves height of the custom shed is lower than that of the standard shed then the struts and cross-bracing for the standard shed are suitable for the custom shed. Another check is to compare the length of the roof slope for the custom shed against that of the standard shed, if the custom roof length is shorter then the roof bracing for the standard shed is suitable for the custom shed. Still further check is to make sure the area of the end wall for the custom shed is less than that of the standard shed, which should be clearly apparent from the custom shed fitting inside the larger shed.

If end wall mullions/columns are also drawn on the template then those required for the custom shed can be compared for length and spacing against those of the standard shed. If the length of the mullions and the spacing between them, are smaller than those of the standard shed, then those of the standard shed are suitable for the custom shed.

Thus it can be seen that the one simple diagram can be used to reach a number of design decisions that would otherwise require a considerable amount of calculation. To assist in producing such proof-templates and associated specifications all the information listed above should be collected from all the available in-house design reports. Plus the following additional information:

Wall Girts

Roof Purlins

Roof Cross Bracing

Wall Cross Bracing

Struts

Connections (Base, Eaves, Ridge)

End Wall Columns (c-section, and connection)

Then consider producing one specification and one proof-template for each of the following:

Gable Sheds (Wind Terrain Category 2)

Gable Sheds (Wind Terrain Category 3)

Skillion Sheds (Wind Terrain Category 2)

Skillion Sheds (Wind Terrain Category 3)

Free standing Gable Roof Canopies. (Wind Terrain Category 2)

Free standing Gable Roof Canopies.(Wind Terrain Category 3)

If a manufacturer/supplier summarises their collection of design reports into a few “proof-templates” and also a few simple specifications, then it is possible to get an independent technical expert (engineer) to certify the summary, having reviewed all the contributing reports.

This summary would then simplify the paperwork required for submission to council, for they should no longer require the contributing reports.

But they will require your proof that what the customer wants is compliant with the range of acceptance defined by your proof-templates. That is for every building application, you need to draw the customers shed on the proof-template and submit it with your application for approval.

Now if the above “proof-templates” demonstrate that a custom design is not compliant with your available standards, or council will not accept the “proof” as an adequate demonstration of compliance, then a custom design (drawings & calculations) will be required. Or additional calculations will be required to produce or extend the range of suitability of the proof-template.

Thus it can be seen that by recording all customers’ requirements on a proof-template, the need for additional calculations can be easily identified. More importantly, the specifications can be seen to be managed, and the calculations requested from an engineer can be more limited in scope that the design of an entire building.

If the manufacturer/supplier goes to an engineer and mentions building, the engineer will start considering the risk, liability and responsibility involved with designing the entire building. If they are shown a drawing of the entire building and are only requested to design a small part of it, they still have a responsibility to question the engineering for the rest of the building.

However, if we concentrate on the analytical and mathematical skills of the engineer, then we can pose a mathematical problem for the engineer to solve, a problem that as nothing to do with any building project. If the problem posed is inappropriate to the building concerned then that is the manufacturer/suppliers risk.

Now one way to pose the mathematical problem is to request an extra proof-envelope for the proof-template. Draw the custom envelope on the existing or new proof-template and request section sizes and calculations to support.

To use the proof-templates effectively it is necessary to develop at least a good qualitative understanding of the behaviour of structures. See future technical notes for guidance in developing this skill.